Flowmeters of the aforedescribed kind are known to the art and can be referred to as "by-pass flowmeters", since a determined part of the momentary total flow through the main conduit is caused to pass through the measuring conduit and the volume of this flow is proportional to the total flow through the main conduit. These known flowmeters include a larger or smaller fixed throttling orifice in the main conduit section, and the measuring conduit, or branch pipe, connected in parallel across the constriction includes a flowmeter which delivers a signal corresponding to the flow (or rate of flow) through the measuring conduit.
For the sake of simplification, this signal is exemplified in the following description as one pulse per unit of volume passing through the measuring conduit.
A measured frequency multiplied by the volume unit can then be considered proportional to the by-pass flow or the percentage of flow through the subflow meter and therewith, with a chosen constant, also proportional to the total flow through the main conduit.
The pressure difference occurring across the throttle will increase with increasing flow through the throughflow area of the throttle and therewith drive the by-pass flow through the measuring conduit and the measuring device and through the main conduit section of the flowmeter.
It is known that a maximized, accepted pressure difference and flow rate in the measuring conduit and through the subflow meter is applicable to a maximum total flow through the main conduit.
It is also known to use the same by-pass meter for different measuring ranges, by appropriately dimensioning the cross-section of the main conduit and the size of the throttle in accordance with a chosen measuring range.
Among other things, it is necessary to afford each flowmeter with the largest possible measuring range or dynamics, as will be explained more precisely in the following.
In the case of a by-pass meter of the type intended here and described in some detail in the aforegoing, it is known that when the resistance coefficient of the throttle and the measuring conduit together with the by-pass meter are equal and the flow is turbulent, the ratio of the flow through the throttle to the flow through the measuring conduit will be constant, and that the signal delivered by the flowmeter in the measuring conduit will be proportional to the sum of both flows.
The British Patent Specification 2257/1886 discloses an arrangement which includes a by-pass meter and a variable throttle. Since this throttle is located downstream of the by-pass meter where the unmeasured main flow and the measured by-pass flow combine to form a common flow, the ratio of the main flow to the by-pass flow is not affected by the throttle. The throttle is obtained with the aid of a pivotally suspended flap which is constructed to maintain a constant proportionality between measured and unmeasured water flows, irrespective of the instant positional setting of the flap.
The flap functions to urge the measuring flow through the by-pass meter, which is comprised of an impeller, in response to the pressure exerted thereon by the main flow, which results in an unequivocal ratio between the occurrent pressure and the generated measuring flow with unchanged low flowmeter dynamics.
Practical tests performed on the known by-pass flowmeters have normally shown a flowmeter dynamic in the order of 50:1. A flowmeter dynamic in the order of 100:1 is most unusual if it can be achieved at all with good linearity while satisfying other general demands.
It is well known to the person skilled in this art that the advantages afforded by by-pass meters of hitherto known construction reside in the possibility to construct large flowmeters adapted to large volumes at low costs, and that one disadvantage of such flowmeters resides in their excessively restricted flowmeter dynamics.
Also belonging to the earlier standpoint of techniques with regard to the inventive flowmeter are flowmeters which function according to other concepts, namely flowmeters which lack a measuring conduit through which a by-pass passes.
According to one embodiment of this latter flowmeter, a restriction is used in the main conduit and flow is measured by sensing the instant pressure difference between occurrent pressures on each side of the constriction.
In a flowmeter of this kind, the flow detected by the meter is proportional to the square root of the pressure difference.
Flowmeters of this category have been found to be highly accurate within a pressure difference range of 50:1, which then gives a flowmeter dynamic of only about 7:1, at a constant throughflow area formed by the constriction and obtained, for instance, by using an orifice plate, a measuring flange, a measuring nozzle, a Venturi tube or some like device.
While taking into account the requirement of a maximized, accepted pressure difference across the constriction, various measures have earlier been proposed for attempting to increase the measuring range of the flowmeter or to increase the flowmeter dynamics (Qmax:Qmin) while maintaining the pressure drop (the pressure losses) through the flowmeter at an acceptable low level.
It is earlier known in this regard to increase flowmeter dynamics with the aid of a throughflow area which is dependent on the instant flow and which is therefore variable. The measuring device connected to the system still measures the flow as a function of the occurrent pressure drop across the constriction.
In the case of such pressure difference flowmeters, it is known to determine the occurrent, instant pressure drop with the aid of a pressure differential transmitter.
It is also earlier known that flowmeter dynamics can be improved when the variable throughflow area is permitted to increase with increasing pressure difference, and vice versa. This is achieved with the aid of an axially movable and spring-biassed throttling body placed in the main conduit, for instance a flow throttling body of the kind illustrated and described in British Patent Specification 1,566,251. In this regard, it has been found that the flowmeter dynamics can be increased to an order of magnitude of 50:1.
Finally, it can be mentioned that it is known that the dynamics of a flowmeters can be increased by up to 50:1 when an inductive transducer is used directly in the main conduit. However, this increase in flowmeter dynamics is obtained at the cost of the possible choice of the flowing medium, since an inductive transducer requires the presence of an electrically conductive medium.
It is also known to detect the output signal from different measuring devices with the aid of electronic signal converters and to convert the signal to a proportional signal corresponding to a total medium flow, in accordance with a mathematical function.
It is possible to introduce minor corrections to the signal with the aid of these electrical signal converters, so as to compensate for minor deviations from sufficiently accurate proportionality.